In a wavelength-division multiplexed (WDM) optical network, independent signals are modulated onto optical carriers in distinct wavelength channels, and concurrently injected in a common optical fiber. Typical wavelength channels of current interest are spaced apart by 25, 50, 75, or 100 GHz. In most current WDM networks, the optical carrier in each band is generated by a separate laser. Since there are typically several tens of wavelength channels, the optical sources for carrier generation contribute substantially to the overall cost of the network. Those skilled in the art have recognized the advantages of a multiple-wavelength optical source that does not require a separate laser for each wavelength channel.
It is known that a laser beam at essentially a single wavelength can be arranged to interact with a frequency-shifting optical element of an appropriate kind to generate a beam containing a sequence, or “comb,” of distinct wavelengths. Such an arrangement is described, for example, in U.S. Pat. No. 5,101,291, which issued on Mar. 31, 1992 to R. M. Jopson (the '291 patent).
The frequency-shifting element of the '291 patent is a Bragg cell. A Bragg cell is an optically polished crystal in contact with an electroacoustic transducer arranged to inject an ultrasonic wave into the crystal. A laser beam passing through the crystal interacts with the ultrasonic wave. In specific directions, the laser beam scatters from the crystal with an optical frequency shift of plus or minus the acoustic frequency of the ultrasonic wave.
In the arrangement of the '291 patent, the Bragg cell is included in an optical ring which also includes an optical amplifier. Light injected into the ring at the laser frequency undergoes a successive frequency shift on each pass through the Bragg cell, followed by amplification to compensate for optical losses in the ring. Each shift adds a further wavelength to the sequence of wavelengths that make up the resulting comb. A bandpass filter included in the ring limits the total number of wavelengths in the comb, so that the available optical power will not be depleted by spreading it over too many wavelengths.
Although useful, the comb generator of the '291 patent has limited application to WDM networks. The reason for this is that it is not generally feasible to generate ultrasonic waves of appropriate properties at frequencies substantially greater than 1 GHz. Consequently, it is not generally feasible to achieve the desired spacings between optical wavelength channels for WDM networks.
Frequency-shifting technologies other than the specific Bragg-cell-based technology of the '291 patent have been described in U.S. Pat. No. 5,734,493, which issued to R. M. Jopson on Mar. 31, 1998, and is commonly assigned herewith. Technologies described there include the bulk magneto-optic Bragg cell, waveguide magneto-optic scattering, waveguide acousto-optic scattering, and single-sideband modulation. Of these technologies, only single-sideband modulation has achieved optical frequency shifts of 20 GHz or more. However, difficulties with suppression of unwanted optical frequencies may limit applications of this technology in WDM systems.
Thus, there remains a need for an optical comb generator capable of achieving channel spacings appropriate for WDM networks.